The present invention relates to propulsion systems and, more particularly, to gear casings for marine propulsion equipment.
One or more gas or steam turbines are favored prime movers in certain marine vessels. There are a number of problems which must be overcome before the shaft output of a gas or steam turbine may be employed to drive the vessel. The principal problem arises from the fact that efficient operation of a gas or steam turbine occurs at a relatively high shaft speed of, for example, about 6000 RPM whereas a conventional marine propulsion screw requires a shaft speed about 25 times slower. Thus, a reduction gearing assembly is conventionally required between the output shaft of the gas or steam turbine and the propeller shaft. Such conventional reduction gearing assembly accomplishes the shaft speed reduction in two stages. The first stage of reduction employs a pinion driven by the gas or steam turbine shaft driving a pair of first stage reduction gears. The second stage of reduction employs second pinions driven by the first stage reduction gears. The second pinions mesh with a second, or final, gear, colloquially known as a bull gear because of its large size. The bull gear is coaxially connected to the propeller shaft.
Designers of marine vessels always place reduction in size and weight of all components in a marine vessel high on their want list. This is especially true of vessels designed for high speed. A casing for containing a gearing assembly is a difficult subject for size and weight reduction because of the inherently large size of the bull gear required to carry the shaft torque and also because the structure of the casing must be rigid enough to withstand torque-derived twist. If such twist is not satisfactorily resisted, distortion of the casing under load may permit a change in the mesh of one or more pairs of meshed gears thereby leading to excessive wear, inefficient power transmission and a large increase in gear noise. In certain noise sensitive naval vessels, the problem of gear noise, leading to noise transmission into the water surrounding the vessel is of considerable concern.
A noise-sensitive vessel, of the prior art, generally are equipped with a large rigid sub-base which is shock-mounted to the frame of the vessel. The turbines and gearing are rigidly mounted on the sub-base. The casing of the gearing assembly, while itself a relatively rigid member, is further strengthened by being rigidly affixed to the sub-base. The sub-base is, however a parasitic mass which does not contribute actively to the propulsion of the vessel. Elimination of the sub-base, or at least a substantial reduction in its parasitic weight, is clearly a desirable objective. In order to achieve this objective, however, it is necessary to offset the loss of structural stiffness which the gear casing formerly received from its rigid mounting on the sub-base.
A further problem with the gear casing of the prior art, and one which is contrary to the desire for increased stiffness, is brought about by the problem of mounting the large bull gear within the casing. The casing is conventionally fabricated in three major welded pieces. A lower casing and an upper casing are split horizontally at a bolted flange passing through the center of the shaft of the bull gear and encircling the entire casing. An oil pan extends downward below the lower casing to cover the lower protrusion of the bull gear extending below the lower casing. The oil pan is attached to the lower casing by a bolted flange.
To install the gearing assembly, the oil pan and lower casing are first installed in the vessel; then the bull gear, with its shafts, is installed on lower halves of journal bearings provided in the upper perimeter of the lower casing. Once the bull gear is in place, the upper casing is installed. The upper casing carries lower halves of journal bearings for shafts of the first stage reduction gears and input pinions. Thus, after the upper casing is in place, the remainder of the gearing, as well as suitable covers, may be installed.
The three separate bolted-together welded pieces of the prior art would not provide the required rigidity if deprived of the support of a massive rigid sub-base.